1. Field of the Invention
The present invention relates generally to an optical fiber link that enables the transmission of dense wavelength-division-multiplexed optical signals across a wide band without significant degradation from non-linear effects, while also permitting Raman amplification within the link. More particularly, the present invention relates to an optical fiber having a low effective area, non-zero dispersion, and low dispersion slope for performing Raman amplification downstream from a fiber of high effective area.
2. Description of the Related Art
Optical transmission systems are continually being sought that will communicate wavelength-division-multiplexed signals over longer distances (several thousand kilometers) without amplification or regeneration, or with amplification or regeneration limited as far as possible, and will handle increased bandwidths. The current amplification band for optical systems is generally about 30 nm wide in the so-called C-band around 1550 nm, but developments are expanding this band to about 70 nm and including the so-called L-band of amplification around 1580 nm.
Non-linear optical effects are known to degrade the quality of transmission along standard transmission optical fiber in certain circumstances. These non-linear effects, which include four-wave mixing, self-phase modulation, Brillouin and Raman scattering, and cross-phase modulation, induce distortion into the transmitted signal in high-power systems, thereby degrading the quality of the transmission. In particular, the non-linear effects can hamper quality transmission using wavelength division multiplexing (WDM), which otherwise greatly enhances the signal carrying capability of optical transmission fibers by increasing the number of transmission channels through which signals may be sent.
These non-linear effects, particularly the phenomenon of four-wave mixing (FWM), can be minimized or avoided by using single-mode transmission fibers that have a large effective area and an absolute value of local dispersion that is greater than zero around the operating wavelengths. In advanced WDM systems, such as Dense Wavelength Division Multiplexing (DWDM) (spacing≦0.8 nm) and Hyper-Dense Wavelength Division Multiplexing (HDWDM) systems (spacing≦0.4 nm), where the transmission channels are closely packed together, the value of local dispersion must meet a minimum value to maintain the quality of the signals. On the other hand, if the dispersion value of the fiber becomes too large, the signals will become distorted during transmission unless dispersion correction devices are included in the transmission line. Thus, for an optical fiber to be effective in a WDM system, the fiber must have a minimum dispersion, but the value of dispersion must also be below a maximum value.
In general, increasing effective area and maintaining low loss and good macrobending/microbending loss behavior yield an increase in dispersion slope. As the bandwidth of WDM communications widens, however, a flat dispersion slope becomes increasingly more important to avoid dispersion variation between channels. In particular, the combination of high dispersion slope, wide WDM band, and long distance gives high differences of accumulated dispersion of the side wavelengths of the WDM band. Even in the presence of exact dispersion compensation at a particular position of the WDM band (e.g. the center channel), the channels set apart from that particular position (e.g. the side channels) will accumulate large amounts of chromatic dispersion. The interplay between dispersion and non-linear effects deteriorates these outlying signals in such a way that it prevents the receiver from recovering them, even in the presence of an optimal channel-by-channel pre-compensation at the transmitter side and/or post-compensation at the receiver side. Consequently, a low slope of dispersion across an operating bandwidth is important for effective transmission of WDM channels in both the C and L bands.
Fibers having low dispersion slope are known. For example, U.S. Pat. No. 5,553,185 to Antos et al. discloses a non-zero dispersion fiber that is characterized by a series of core regions that have different refractive-index profiles and widths. The shape of the refractive-index profiles, in terms of the refractive-index difference and the radius, of each region may be adjusted to have properties tailored for a high performance telecommunication system. In particular, one of the regions has a depressed refractive-index difference. The dispersion slope of the disclosed fiber is less than 0.05 ps/nm2/km and the absolute value of the total dispersion is between 0.5 and 3.5 ps/nm/km over a pre-selected transmission range, which spans about 1490-1575 nm. In addition, the zero-dispersion wavelength is about 1593 nm, which is outside the pre-selected transmission range.
Another fiber for a high performance communication system is discussed in Y. Akasaka et al., Enlargement of Effective Core Area on Dispersion-Flattened Fiber and Its Low Non-Linearity, OFC '98 Technical Digest, pp. 302-304. This fiber is also characterized by a series of core regions having varying refractive-index differences and radii. One of the core regions also has a depressed refractive-index difference. The disclosed fiber has a dispersion slope lower than standard single-mode fiber over the transmission window.
Distributed Raman amplification in the transmission line may be used, e.g., to increase the transmission distance between erbium-doped amplifiers, and/or to reduce the span loss that would result from an increase in the number of WDM channels or a decrease in power and cost reduction of the erbium doped amplifiers. Stimulated Raman scattering is a nonlinear process that can cause an optical fiber to amplify an optical signal. The Raman scattering converts a small fraction of the incident power from a pump optical beam to another optical beam at a frequency downshifted by an amount determined by the vibrational modes of the medium. The usual configuration of distributed Raman amplifier uses the transmission line as the medium wherein a pump, which typically is counter-propagating, causes stimulated Raman scattering at the signal wavelength. Two “rate equations” govern the dynamic of the signal (PS) and pump (PP) powers in the optical fiber in the case of a counter-propagating pump:                                           ⅆ                          P              S                                            ⅆ            z                          =                                                            g                R                                            a                P                                      ⁢                          P              P                        ⁢                          P              S                                -                                    α              S                        ⁢                          P              S                                                          (        1        )                                                      ⅆ                          P              P                                            ⅆ            z                          =                                                            ω                P                                            ω                S                                      ⁢                                          g                R                                            a                S                                      ⁢                          P              P                        ⁢                          P              S                                +                                    α              P                        ⁢                          P              P                                                          (        2        )            where αS and αP are the attenuation coefficients of the optical fiber at signal and pump wavelength respectively, ωS and ωP are optical frequencies of signal and pump wavelength respectively, aS and aP are the effective core areas of the optical fiber at signal and pump wavelength respectively and gR is the Raman-gain coefficient of the fiber material for a given couple of optical frequencies. See Agrawal, Nonlinear Fiber Optics, 2nd edition, ch. 8 (1995).
In general, distributed Raman amplification operates in an unsaturated regime, wherein signal power levels are maintained well below pump power levels. Under this assumption, one can neglect pump depletion, i.e., the first term in the right hand side of (2). Thus Equations (1) and (2) can be easily solved to obtain the signal power:                                           P            S                    ⁡                      (            L            )                          =                                            P              S                        ⁡                          (              0              )                                ⁢                      exp            ⁡                          [                                                                                          g                      R                                                              a                      P                                                        ⁢                                                            P                      P                                        ⁡                                          (                      L                      )                                                        ⁢                                      L                    eff                                                  -                                                      α                    S                                    ⁢                  L                                            ]                                                          (        3        )            where L is the length of the medium fiber, PS(0) is the input signal power, PP(L) is the input (counter-propagating) pump power at the opposite end of the fiber, and Leff is the effective interaction length at pump wavelength given by:                               L          eff                =                                            1                              α                P                                      ⁡                          [                              1                -                                  exp                  ⁡                                      (                                                                  -                                                  α                          P                                                                    ⁢                      L                                        )                                                              ]                                .                                    (        4        )            A Raman amplifier is characterized in terms of (linear) amplifier gain or ON/OFF ratio, which is defined by the following:                                           G            A                    ⁡                      (            L            )                          =                                                            P                S                            ⁡                              (                L                )                                                                                      P                  S                                ⁡                                  (                  0                  )                                            ⁢                              exp                ⁡                                  (                                                            -                                              α                        S                                                              ⁢                    L                                    )                                                              =                      exp            ⁡                          [                                                                    g                    R                                                        a                    P                                                  ⁢                                                      P                    P                                    ⁡                                      (                    L                    )                                                  ⁢                                  L                  eff                                            ]                                                          (        5        )            Typical values of PP(L) range from 100 mW up to 1000 mW and above. Moderate values of pump power are considered less than 500 mW.
From Equation (5), one can determine that an efficient fiber in terms of distributed Raman amplification has the following properties:                small effective area (less than 45 μm2) at the pump wavelength (typically between about 1400 nm and 1510 nm, preferably between 1440 nm and 1490 nm) or equivalently a small mode field diameter at the pump wavelength; and        long effective length at the pump wavelength, which is equivalent, for a fixed length L of the fiber medium, to a low loss αP at the pump wavelength.These observations are well in agreement with experimental results shown in the article Fludger et al., “An analysis of the improvements in OSNR from distributed Raman amplifiers using modern transmission fibres,” OFC2000, FF2-1, pp. 100-102 (2000).        
In the following, we will refer to an exemplary pump wavelength of about 1450 nm, but the invention contemplates the use of pump wavelengths from about 1350 nm to 1510 nm. In general, a fiber having small effective area at 1450 nm also exhibits a small effective area at the conventional transmission bands C and L. This requirement conflicts with the need for fiber with high effective area for avoiding non-linear effects in dense WDM communications.
Besides low effective area, it is well known that other important conditions must be taken into account to achieve acceptable Raman amplification. Most important are the double backscattering of the amplified signal and the single backscattering of the spontaneous emission of the pump signal, as explained in Nissov et al., “Rayleigh crosstalk in long cascades of distributed unsaturated Raman amplifiers,” Electronics Letters, vol. 35, no 12, pp. 997-998 (Jun. 10, 1999); Wan et al., “Impact of Double Rayleigh Backscatter Noise on Digital and Analog Fiber Systems,” Journal of Lightwave Tech., vol. 14, no. 3, pp. 288-297 (March 1996); and Hansen et al., “Rayleigh Scattering Limitations in Distributed Raman Pre-Amplifiers,” IEEE Photonics Tech. Letters, vol. 10, no. 1, pp. 159-161 (January 1998).
In general, when a system uses distributed Raman amplification, there is an optimal value of pump power above which the performance is degraded by detrimental effects due to single and double backscattering., as shown in Garrett et al., “Field demonstration of distributed Raman amplification with 3.8 dB Q-improvement for 5×120 km transmission,” OFC2000, PD42-1 to PD42-3 (2000).
In conclusion, a fiber suitable for a system having distributed Raman amplification is characterized by:                small effective area at the pump wavelength, in order to have high Raman gain efficiency;        low loss at the pump wavelength, in order to increase the interaction length between pump and signal;        low loss at the signal wavelength, in accordance with Equation (3);        low back-scattering at the signal wavelength, in order to reduce noise; and        single mode performance at the pump wavelength when in cable form, in order to confine pump power into the fiber core.        
The fiber loss is the sum of different attenuation mechanisms (such as infra-red absorption, OH absorption, UV absorption), among which a major role at operating wavelength is played by Rayleigh scattering and other scattering mechanisms. Backscattering is the combination of an elastic scattering process along the fiber (typically, although not restrictively, Rayleigh backscattering) and the mechanism of recapture of light in the backward direction. In general, for common transmission fibers high loss implies high backscattering, because the dominant term of loss is the elastic scattering of the light.
A few publications address a combination of optical fibers in a transmission link that includes Raman amplification. WO99/57822, for example, discloses a unitary, dispersion-managed optical fiber that has a total dispersion that changes from positive to negative along the length of the fiber over a transmission wavelength range. The fiber may include distributed amplification by stimulated emission of a rare-earth dopant, Raman effects, or by both. An embodiment in WO99/57822 contemplates shorter sections of the unitary fiber having higher loss, especially when the shorter sections are co-doped with alumina and have a smaller effective area for better pump light/erbium overlap. In this embodiment, the longer sections would be designed to have lower loss and larger effective area. In a preferred embodiment, sections having negative dispersion also have negative dispersion slope. Applicants has observed that the latter generally causes high loss due to high fluorine dopant content.
WO99/66607 discloses an apparatus and method for combining optical amplification and dispersion compensation in a Raman amplifier. A device called a dispersion-managing Raman amplifier (DMRA) combines Raman amplification and has its length and dispersion selected to balance the dispersion of a link. Used as a dispersion compensator that aims to reduce total dispersion to zero, the DMRA has high negative dispersion and a low effective area, which enables Raman amplification.
U.S. Pat. No. 5,191,631 describes a hybrid optical fiber that has a desirable fiber dispersion characteristic and a relatively large effective area. Hybrid fibers are constructed by splicing together a fiber with a large effective area but typically unsatisfactory fiber dispersion characteristics to a fiber having excellent dispersion characteristics but a smaller effective area. The patent, however, does not contemplate having distributed Raman amplification across the second fiber, having a low dispersion slope in the second fiber, or obtaining a low slope of dispersion across the link for WDM applications.
Applicants have observed the need for an optical transmission link that permits the transmission of dense WDM channels across a wavelength band that includes the C-band, and may also include the L-band, without destructive FWM and that includes distributed amplification within the line.